Sticker, answers to questions:
Volume at 1.8 was fairly loud
The led is a power on indicator, off for fault, overheat, etc
Main power voltage is 14.1, no power on lead at the 404. It detects high level and turns on.
The high output wires are crimped into adapter leads. I didn't cut vehicle wires. Easier to return to stock.
The MTX 404 is new.
And, yes, the sub plays, via high level when I bypass.
So, looks like next step is to return the 404 and try a replacement.
Thanks for your help.